In the field of utility meter reading, wireless communication networks are implemented to gather consumption data from the meters. Generally, a utility meter is equipped with an end point device that accumulates data based on consumption through the meter. This data is then transmitted over a radio frequency (RF) network for receipt by a remotely located station or unit. Depending on the system structure, this data then can be processed and/or re-transmitted to another remote station or unit. While meter reading networks will be referenced as examples of where the present invention can be implemented, the present invention can be employed in any wireless system where low cost, low power, transceivers are needed.
In a metering wireless configuration, unlicensed transceivers using spread spectrum technology must necessarily operate under low power requirements, as dictated by Federal Communications Commission (FCC) regulations. In addition, low power transceiver designs are essential since the transceivers will often be encompassed within handheld portable units, or attached to end point devices. In either case, power conservation issues are a priority since the transceiver, and the other circuitry within the units, derive power from a limited electrical power source such as a battery. As a result of these power concerns, it is desirable to minimize needless transmissions, complex circuitry, and repetitive processes in order to promote the most efficient use of the available power. Ideally, this can be done while, at the same time, minimizing costs.
In wireless networks, there are often significant periods of time when low power transceivers are not required to transmit data. To conserve valuable power, a power reserving “sleep mode” is triggered. In sleep mode, the transceiver awaits a polling signal. The polling signal awakens the transceiver for data transmission. Generally in this sleep mode, any circuitry that is not needed to receive the polling signal is powered down until needed upon transmission initiation.
Spread spectrum technology is widely utilized in wireless networks since it provides additional security, licensing benefits under FCC rules, and resistance to interference. A spread spectrum communication system transmits signals over bandwidths much larger than those actually required to transmit the information. There are two forms of spread spectrum communication utilized in conventional meter reading networks: direct sequence spread spectrum (DSSS) and frequency hopping spread spectrum (FHSS).
In a DSSS system, a PN spreading code generator is used to modulate a frequency carrier. The bandwidth of a DSSS system is a derivative of the chip rate. In an FHSS system, the carrier frequency of the transmitter changes in accordance with the PN spreading code, with the receiver continuously changing its frequency based on its complimentary PN spreading code. The order in which frequencies are occupied are a derivative of the code, and the rate of frequency hopping is a function of the chip rate.
FHSS systems have proven especially appealing in transmitting commodity consumption data in meter reading networks since short bursts of data are transmitted at a rate greater than that obtained under a DSSS transmission. Additionally, the ability to jump between multiple frequencies significantly reduces the chance of interference with an FHSS system. However, the functional benefits associated with FHSS systems inevitably prove problematic in wireless networks requiring low cost, low power, transceivers.
FHSS systems require transceivers with relatively complex circuitry and power consuming circuitry. Specifically, if not controlled, the transmitter will eventually wander or drift in frequency, producing unpredictability in the transceiver operation. To combat this problem it is necessary to stabilize frequency drifting. A phased lock loop is typically used to provide this frequency stability, but such locking circuitry is expensive and results in an undesirable drain on power. Therefore, while the transmission benefits of an FHSS system are appealing, it is necessary to allocate power in a manner that maximizes the efficiency in which the system taxes valuable battery power.
DSSS systems also provide benefits for use in a wireless communication network. As relative to the present invention, the transmission of data over a DSSS signal does not necessarily require the use of locking circuitry since frequency stability is not as significant of an issue. A DSSS transceiver is capable of communicating over a relatively “sloppy”, single frequency signal. This single wideband signal significantly reduces the possibility of a transceiver being falsely awakened from deep sleep mode by random noise, which is a problem with narrow band transceivers, as used in an FHSS system. Specifically, the DSSS spreading code alone can serve to awaken the transceiver. While data can be decoded, it is not required to just awaken the transceiver. For these reasons, the problems often associated with frequency stabilizing and powering up are avoided. However, as stated previously, DSSS systems transmit at a lower data rate than FHSS systems. In addition, it is more difficult and expensive to decode the data encoded within a DSSS signal. Since information within the DSSS signal is spread out over a wide bandwidth in a single transmission, decoding or “de-spreading” of the information upon receipt requires a relatively complex decoder. Similarly, a complex encoder is needed to attach data to a DSSS signal. Moreover, the use of a single transmission frequency introduces another drawback in a wireless communication network. The use of a predetermined frequency increases the chances of interference, thus requiring innovations in network structure and transmission timing to better ensure that data transmissions are not lost.
In light of the strengths and weaknesses of DSSS and FHSS systems, a standard practice has been to go with one system protocol over the other, depending upon the particular balance of cost and performance for a given application. In the field of meter reading specifically, the choice has typically been to implement an FHSS system. In a wireless network where a plurality of end point devices periodically transmit short bursts of data to a plurality of remote receiving units, the signal interference benefits and increased transmission rates associated with an FHSS system have made it preferable over DSSS. However, a “pure” FHSS system is problematic since it must keep power demanding circuitry running in order to receive a polling signal. For this reason, an FHSS transceiver cannot go into true deep sleep mode to most efficiently preserve power. Conversely, a DSSS transceiver does not need to maintain stringent frequency accuracy, and without the need for complex frequency locking or decoding circuitry during the polling process, it is able to better optimize power conservation during deep sleep mode.
U.S. Pat. No. 5,661,750 ('750) does describe a system for utility metering implementing DSSS technology, where the system is designed to utilize a high power transmitter and still meet FCC requirements. Specifically, in the '750 system, the transmitter utilizes a modulator to modulate the transmission signal with a pseudo-random pattern to spread the signal across a broader bandwidth than the original signal and uses a second modulator to modulate a preamble of the signal with a phase reversal pattern. The phase reversal pattern increases the number of spectrum lines produced by the transmitter and thereby decreases the power density of the broadcast signal, which for DSSS is +8 dBm in any three KHz bandwidth. While such a DSSS system is recognized to have benefits, the '750 invention uses DSSS transmissions indiscriminately, and in particular, it uses DSSS during the transmission of substantive data rather than as an efficient transceiver wake-up technique. Moreover, the use of DSSS technology for transmitting end point data does not address the signal collision avoidance inherent within an FHSS system.
One system encompassed within the Inovonics TapWatch□ system has utilized both DSSS and FHSS technologies in one meter reading network. However, the two spread spectrum technologies are implemented at separate system points within the larger network and have not been optimally combined within any single transceiver in the system so that two communication nodes or points on the network can communicate with each other using both DSSS and FHSS. With Inovonics, the end point transmitters attached to each utility meter utilize a low power FHSS transmitter having less than 0.5 mW. A network of intermediate repeaters receive the low power FHSS transmissions from the end point transmitters and convert these transmissions to DSSS transmissions that are retransmitted by a high power transmitter operating under FCC regulations to base stations for collection and processing. Transmissions between the end point transceiver and the intermediate repeaters are always done over an FHSS signal, and transmissions between the intermediate repeaters and the base station are always done over a DSSS signal.
Based on the inherent advantages and disadvantages of both the DSSS and FHSS systems, there is a need for a low power transceiver that incorporates the benefits from both. While an FHSS system is preferable at the data communication or transmission stage, the power conservation focus of a deep sleep mode is more efficiently dealt with under a DSSS system. The present invention advances a low power transceiver that utilizes DSSS technology for waking up from deep sleep mode and FHSS technology for substantive data communication with the remote receiving unit. The communication between the end point devices and the remote receiving units is not limited to one spread spectrum protocol.